Generally, construction machines like hydraulic excavators have an upper revolving structure rotatably mounted on a vehicular lower structure, the upper revolving structure being put in swing motions under control of an inertial body drive system which is adapted to suppress a reversing movement which occurs to the upper revolving structure upon stopping a swing motion (e.g., see WO 94/01682 or Japanese Patent Publication Gazette No. H8-6722).
The inertial body drive system of the prior art of this sort is provided with an anti-reversing spool valve and a pressurized oil supply means between first and second main conduits which are connected to a swing-drive hydraulic motor. As a braking pressure is dropped by opening and closing operations of an overload relief valve during an inertial rotation of an upper revolving structure of an excavator, the spool valve is temporarily opened by a pressurized oil (a pilot pressure) which is supplied from an oil reservoir chamber of the pressurized oil supply means.
As a consequence, the above-mentioned first and second main conduits are brought into communication with each other through the spool valve, a pressure difference between these two main conduits is abruptly diminished, bringing the swinging hydraulic motor to a stop and suppressing a reversing movement of the upper revolving structure which would normally occur upon stopping an inertial rotation.
In the above-mentioned prior art, an anti-reversing spool valve is directly connected between the first and second main conduits, and a throttle as a flow resisting means is provided in the way of a conduit which connect an oil reservoir chamber of the pressurizing oil supply means with a tank, thereby controlling the length of an open time period of the spool valve by way of a sectional flow passage area of the throttle.
The spool valve is provided with a valve spring which is adapted to bias a spool constantly toward a closed position. For this valve spring, a relatively weak spring is employed for the purpose of prolonging an open time period of the spool valve. In addition, a sectional flow passage area in the above-mentioned throttle is reduced for the purpose of elongation of an open time period of the spool valve.
However, when stopping a swing motion of an upper revolving structure in a cold district where operating oil is at a higher viscosity due to low ambient temperature, the flow rate of operating oil through the throttle drops to a smaller rate to elongate the open time period of the spool valve. Therefore, in a cold district, it takes an extra time for the spool valve to return from an opened position to a closed position at the time of stopping a swing motion of an upper revolving structure of a hydraulic excavator.
In this connection, the open time period of the spool valve is determined in relation with the inertial mass (inertial energy) of the upper revolving structure. Namely, there is a conspicuous difference in inertial mass between an upper revolving structure which is loaded with a large amount of soil in a bucket and an upper revolving structure which is barely loaded with soil (or which is loaded with no soil) in a bucket.
When the inertial mass of an upper revolving structure is relatively large, it is necessary to elongate the open time period of the above-mentioned spool valve to absorb energy of an inertial rotation to a sufficient degree for the purpose of suppressing a reversing movement of the upper revolving structure. However, if a sectional flow passage area of the above-mentioned throttle is reduced for elongation of the open time period of the spool valve, it will give rise to a following problem in case an upper revolving structure of an excavator is small in inertial mass.
Namely, when an upper revolving structure is small in inertial mass, the spool valve is kept open even after it has absorbed energy of an inertial rotation, as a result taking an unnecessarily long time in stopping the upper revolving structure and causing a delay of its stoppage.